1. Field of the Invention
The present invention relates to a superregenerative receiver. More specifically, the present invention relates to an improved superregenerative receiver for use in wireless receivers of a remote control system, citizen band receivers and the like.
2. Description of the Prior Art
A prior art superregenerative receiver is illustrated in a block diagram of FIG. 1. The superregenerative receiver is generally designated by the reference numeral 1 in FIG. 1. The superregenerative receiver 1 includes an antenna 2 and a front end 3 having a superregenerative detector responsive to an incoming signal received at the antenna 2. The front end 3 comprises a buffer amplifier 31 for amplifying the received signal, a quenching oscillator 32 and a low-pass filter 33. As is well-known in the art, the quenching oscillator 32 also serves as a detector and the output of the quenching oscillator 32 is derived as a low frequency signal through the low-pass filter 33. The low frequency signal from the low-pass filter 33 is then fed to a signal level detector 6 through a low frequency amplifier 4 and a bandpass filter 5. When the resultant low frequency signal is above a given level, the signal level detector 6 energizes a buzzer 7. The buzzer 7 thus provides an audible indication that the signal has been introduced into the receiver 1.
The prior art front end 3 will now be described in more detail by reference to FIGS. 2 to 4. The reference numerals 31, 32 and 33 in FIG. 2 represent the buffer amplifier, the quenching oscillator and the low-pass filter, respectively, in the same manner as in FIG. 1. The buffer amplifier 31 includes a transistor T1 which builds up a tuned amplifier of a base-grounded type. The transistor T1 is connected to a power line 30 leading to a power supply +V through a tuning circuit 311 and a resistor. The tuning circuit 311 includes a parallel circuit of an inductor L1 and a capacitor C1 with its tuning frequency in agreement with the carrier frequency of the incoming signal. The incoming signal is received by the antenna 2 (FIG. 1) and fed to the amplifier or the transistor T1 via a terminal A and a coupling capacitor C2. The output of the tuning circuit or a resonance circuit 311 is coupled via the coupling capacitor C3 with the collector of a transistor T2 which constitutes the quenching oscillator 32. The collector of the transistor T2 is connected to the power line 30 via a tuning circuit 321 and an integration circuit 322. The tuning circuit 321 comprises a parallel combination of an inductor L2, a capacitor C4 and a trimmer capacitor C5, while the integration circuit 322 comprises a resistor R1 and a capacitor C6. Connected between the collector and emitter of a transistor T2 is a capacitor C7 which establishes a positive feedback loop therebetween. The base of the transistor T2 is supplied with a base bias voltage which is derived by dividing the power supply voltage +V with a pair of resistors R2 and R3. An oscillating inductor L3 and a parallel circuit of a resistor R4 and a capacitor C8 are connected in series between the emitter of the transistor T2 and the ground. The output of the quenching oscillator 32 is derived from the series junction B of the tuning circuit 321 with the integration circuit 322 to an output terminal C through the low-pass filter 33. The operating principle of the quenching oscillator 32 will be discussed below.
In other words, the quenching oscillator 32 operates in the following manner. Assume now that the transistor T2 is in transition from its conductive state to its non-conductive state. Under the circumstance the collector voltage of the transistor T2 increases gradually according to the charging time constant of the integration circuit 322 as determined by the capacitor C6 and the resistor R1. The varying collector voltage of the transistor T2 is fed to its emitter via the capacitor C7. If the collector voltage reaches its maximum i.e. if current flowing through the oscillating inductor L3 is reduced to a minimum, then the base of the transistor T2 is supplied with the bias voltage in a sense to turn that transistor T2 conductive by virtue of counterelectromotive force developed by the oscillating inductor L3. The transistor T2 therefore becomes conductive abruptly. Once the transistor T2 has become conductive, the oscillating inductor L3 causes counterelectromotive force to render the transistor T2 non-conductive. For this reason the transistor T2 becomes non-conductive and the collector voltage of the transistor T2 rises gradually by the action of the integration circuit 322. In this manner, the transistor T2 is repeatedly switched between the conductive and non-conductive states so as to initiate oscillation. In response to the switching of the transistor T2 between the conductive and non-conductive states, the tuning circuit 321 produces a transient variation in voltage and current. Under these circumstances, the output of the buffer amplifier 31 is applied to the tuning circuit 321 to perform a sort of mixing operation. A modulated signal originating from this mixing operation is supplied via the junction B to the low-pass filter 33.
FIG. 3 shows the waveforms of voltages appearing at the respective nodes A, B and C in FIG. 2 in the absence of the incoming signal and FIG. 4 shows the same in the presence of the incoming signal. It is clear from FIG. 3 that, when no incoming signal is received, only a noise component appears at the output terminal C of the low-pass filter 33 and the bandpass filter 5 (FIG. 1) removes such noise component. At this moment, the buzzer 7 is never enabled with the signal level detector 6.
If an incoming signal is received by the antenna 2, then the incoming signal is admitted to the buffer amplifier 31 in the form of a waveform of FIG. 4A. The voltage level of the received signal is amplified by the buffer amplifier 31 and supplied to the quenching oscillator 32. The quenching oscillator 32 develops its oscillation output voltage as indicated in FIG. 4B. Within the quenching oscillator 32 the received signal is mixed into the resultant oscillation signal and the low frequency signal of FIG. 4C is delivered through the output terminal C of the low-pass filter 33. The low frequency signal is then fed to the signal level detector 6 through the amplifier 4 and the bandpass filter 5 (FIG. 1).
It is generally known in the above described type of superregenerative receiver that sensitivity is under the influence of the oscillation output voltage and the oscillation frequency of the quenching oscillator. With the maximum of the output voltage of the quenching oscillator and the minimum of the frequency of the output thereof, the sensitivity of the receiver is also the maximum. Attention should be paid in this respect at the stage of design of the receiver and the base voltage of the transistor or other factors are selected such that the quenching oscillator provides its maximum output voltage and its minimum frequency. For example, while considering the base voltage of the transistor, there is a very small difference between the maximum voltage (e.g., VB1) at which the maximum output voltage of the oscillator is available and the minimum base voltage (e.g., VB0) at which the oscillator fails to operate. Thus, in the event that the base voltage is selected to be VB1 at the stage of design, there is a possibility that the base voltage will decrease below VB0 due to variations in an ambient temperature, circuit components or power supply voltages. As a matter of fact, the prior art superregenerative receiver is designed with a decrease in sensitivity to the extent that it can provide a stable output despite fluctuations in the abovementioned factors. The prior art superregenerative receiver is therefore unable to make the best of the fact that it essentially exhibits a high sensitivity.